Ultrasonic flow meters have many advantages over other methods of determining flow rates. Ultrasonic flow meters can continuously measure the flow rate, while other methods generally measure average flow rates. In addition, ultrasonic flow meters are obstructionless and work with non-conductive fluids.
Ultrasonic flow meters have a pair of transducers that are placed on either side of the flow path of a fluid flowing through a pipe. The transducers are pointed at each other and placed on either side of the flow path of a fluid flowing through a pipe. The line between the transducers has a component in the direction of the fluid flow. The principle used to detect flow rates is that the transit time of an ultrasonic packet will increase in the upstream and decrease in the downstream path. The amount by which the transit time changes is directly proportional to the flow rate. Generally, the signal is detected by setting threshold and determining when the signal exceeds the threshold. Once the threshold is exceeded the signal is detected at the next positive going, zero threshold. Unfortunately, noise can cause the signal to exceed the threshold on a first cycle or after the first cycle. As a result the ultrasonic flow meter is detecting the received signal at different points along the signal. This result is inconsistent and erroneous transit time information. Automatic gain control circuits are used to overcome this problem. An automatic gain control circuit adjusts the overall signal so that it has a constant amplitude. However, the automatic gain control circuit only adjusts the overall signal level and noise can still cause the signal to cross the first threshold level at different points along the signal.
Thus there exists a need for a method an apparatus that can detect the signal at the same point along the signal consistently.